Sierra Nevada Network

Stressor: Fire

Altered Fire Regime

NPS staff monitor a lightning-ignited fire in the Mineral King area of Sequoia National Park.

From the late 1890s through 1960s, Sierra Nevada park and national forest personnel attempted to suppress all fires, and these efforts were mostly successful. Consequently, numerous ecosystems that had evolved with frequent fires have since experienced prolonged periods without fire.

Change in fire regime has modified ecosystems. In foothill grasslands for example, lack of fire encourages dominance by non-native invasive grasses. Reproduction of shade-intolerant species (e.g., giant sequoia) has been reduced. More land is dominated by dense, intermediateaged forest patches, and less by young patches. Forests are denser, dominated by shade-tolerant species, and shrubs and  herbaceous plants may be less abundant. A buildup of surface fuels has accumulated and increasing numbers of small trees have created "ladder fuels", which carry fire into mature tree crowns. These changes have led to a higher risk of high-severity wildfires than was present before European settlement and fire suppression activities.

Lack of fire can affect water resources by reducing stream flows, altering biogeochemical cycling, and decreasing nutrient inputs to aquatic systems. Less frequent but higher  severity wildfires can also impair water resources, resulting in loss of vegetation cover, litter, and organic matter. The formation of these water repellant soil layers can affect evapotranspiration, infiltration, and snowmelt patterns. Potential impacts include increased flooding, erosion, sediment input, water temperatures, and nutrient and metal concentrations.

Lack of fire has reduced habitat (and food) critical for some wildlife species. Number and extent of forest openings have been reduced, which in turn has reduced key herbaceous and shrub species (e.g., nitrogen fixers such as Ceanothus). Wildlife that require these plants, such as deer, now have less habitat available.

In 1968 (Sequoia & Kings Canyon) and 1970 (Yosemite), NPS staff began prescribed burning. After more than 30 years of prescribed fires, significant progress has been made, although park efforts are far from restoring natural fire regimes at the landscape level.

This article is an excerpt from the Sierra Nevada Network: Vital Signs Monitoring Plan (2007).